Electric generators include a rotor and a stator. Rotors are generally constructed from a steal forging and include a number of slots that run the length of the rotor. Rotors are electrically wound by placing conductors referred to as rotor windings into the slots of the rotor.
Stators are generally constructed from a number of stacked, metal laminations. Stators also include slots, which run the length of the stator. Stators are electrically wound by placing conductors known as stator coils into the slots of the stator.
Conventional stator coils are often held in place in stator slots using a wedge and ripple spring configuration. In this configuration, a stator coil is placed into a slot, and a wedge is driven into groove near the top of the slot. A ripple spring is positioned above the stator coil and below the wedge. This ripple spring provides compressive force to keep the stator coils held firmly in the slot.
Over time, stator wedges may become loose. If a stator wedge becomes loose, it can permit a stator coil to vibrate, which can cause catastrophic failure in an electric generator. In order to avoid stator-coil vibration and catastrophic failure of a generator, it is desirable to periodically inspect the tightness of ripple springs. However, such inspections present a challenge because ripple springs are difficult to access within a generator and because they are concealed by the corresponding stator wedge.
There are a number of conventional approaches to inspecting the compression of ripple springs. One approach involves manually tapping the stator wedges. Another approach involves measuring the depth of the surface of ripple springs through pre-formed test holes in the wedge. A third approach involves physically displacing the wedge and measuring the resulting wedge movement.
There are significant challenges associated with the conventional approaches to testing ripple-spring tightness. The first approach, manually tapping stator wedges, is extremely subjective. The results vary greatly between different inspectors. Manually tapping stator wedges is also only possible after a generator's rotor has been removed from the generator.
The second approach, using a depth gauge to take measurements through pre-formed test holes, is time consuming. This approach is also only possible when a generator has pre-formed test holes in its stator wedges. Many generators do not have such pre-formed test holes.
The third approach, physically displacing the stator wedge, involves impacting a stator wedge and then measuring the displacement of the stator wedge with a sensor such as an optical or capacitive sensor to give an indirect indication of the compression of the ripple spring beneath the stator wedge. This method is not ideal because it involves only an indirect indication of ripple-spring compression. This approach also requires a relatively complex algorithm for converting the displacement of the stator wedge into an indication of ripple-spring compression. U.S. Pat. No. 5,295,388 to Fischer et al, which is incorporated by reference herein in its entirety, discloses a method and system that utilizes this approach.
Despite advances in the area of ripple-spring compression assessment, improved methods and systems are still needed to enable fast, accurate, and direct measurement of ripple-spring compression in generators that do not necessarily have pre-formed test holes.